6
\$\begingroup\$

In a transformer circuit we know that if the secondary side is an open circuit then there will be no current on that side. Therefore, no current can flow on the primary side either because of the relationship

$$i_1\frac {N_1} {N_2}=i_2$$

For this reason, would it be possible to create a "transformer switch" that could stop existing current when connected to a network?

enter image description here

\$\endgroup\$
5
  • 2
    \$\begingroup\$ Current will still flow on the primary side - that creates the magnetic flux. You should do some testing. \$\endgroup\$
    – Solar Mike
    Commented Nov 9 at 14:55
  • 5
    \$\begingroup\$ You're confusing an ideal transformer with a real transformer. \$\endgroup\$
    – Hearth
    Commented Nov 9 at 14:56
  • 2
    \$\begingroup\$ What do you mean by "transformer switch"? How would it "stop existing current when connected to a network", can you explain what do you mean by this too? \$\endgroup\$
    – Justme
    Commented Nov 9 at 15:00
  • \$\begingroup\$ "Therefore, no current can flow on the primary side" - Your basic premise is incorrect. When the secondary is open circuited, the transformer primary appears to the primary side circuit as a simple inductor. Both AC and DC can move through it. \$\endgroup\$
    – AnalogKid
    Commented Nov 9 at 17:52
  • \$\begingroup\$ A transformer with an open circuit on one side is an inductor. You could use an inductor, and you could even short-circuit the inductor to allow more current to flow. However, it doesn't block current completely. \$\endgroup\$
    – Criticizing Israel not allowed
    Commented Nov 9 at 18:04

5 Answers 5

Reset to default
10
\$\begingroup\$

Before powerful valves and semiconductors were available, there was a whole class of amplifier called magnetic amplifiers.

These did not use switching on the secondary side, but auxiliary DC control windings to saturate the core and therefore reduce the inductance, increasing the magnetising current by an order of magnitude or two.

They were still used until quite recently, as they are incredibly rugged and reliable.

\$\endgroup\$
9
\$\begingroup\$

Yes, for example:

schematic

simulate this circuit – Schematic created using CircuitLab

However, the current with SW1 open will not be zero for a real transformer- it will be the magnetizing current. Since it's mostly inductive the wasted energy will not be high though.

When SW1 is closed there will be some voltage drop across the transformer due to the resistance of SW1 and wires and the secondary resistance (multiplied by the transformer ratio) and the primary resistance - and imperfect coupling.

\$\endgroup\$
3
  • \$\begingroup\$ I don't know if they're still available, but there's a type of relay that works on a similar principle. The coil and contacts are mains powered, but there's a separate coil that allows the relay to be switched on and off by a low-voltage circuit. Typical application would be a heater control with low-voltage wiring to a thermostat. \$\endgroup\$
    – Dave Tweed
    Commented Nov 10 at 16:13
  • \$\begingroup\$ @DaveTweed I vaguely remember reading about those, never saw one. Maybe like this? Electric domestic heating was a short-lived trend in this area. \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented Nov 10 at 17:34
  • \$\begingroup\$ No, that's a relay that uses low-voltage bimetallic strips (in order to get a time delay) to activate the contacts, along with a separate power transformer. Not the same idea at all. And this would be for any kind of fuel. Oil burners and some types of forced-draft gas burners require mains AC power, too. \$\endgroup\$
    – Dave Tweed
    Commented Nov 10 at 22:28
6
\$\begingroup\$

would it be possible to create a "transformer switch" that could stop existing current when connected to a network?

I've shown your simplified circuit with the added magnetization inductance in red: -

enter image description here

So, if the secondary is open circuit, current still flows through the primary side via \$L_M\$.

The extent of that current flow is determined by operating frequency and transformer construction/type.

\$\endgroup\$
0
\$\begingroup\$

Yes, the transformer with an open circuit-secondary will reduce the current in anything in series with the primary to the value of the transformer magnetizing current. But that's an expensive way to control current to a device. And you still would need a switch in the secondary to control the current.

\$\endgroup\$
0
\$\begingroup\$

You can switch high current over (very) long distances with a transformer. For this the secondary has to have many more windings than the primary.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.